fsck: document the new receive.fsck.<msg-id> options

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2197,6 +2197,20 @@ receive.fsckObjects::
 	Defaults to false. If not set, the value of `transfer.fsckObjects`
 	is used instead.
 
+receive.fsck.<msg-id>::
+	When `receive.fsckObjects` is set to true, errors can be switched
+	to warnings and vice versa by configuring the `receive.fsck.<msg-id>`
+	setting where the `<msg-id>` is the fsck message ID and the value
+	is one of `error`, `warn` or `ignore`. For convenience, fsck prefixes
+	the error/warning with the message ID, e.g. "missingEmail: invalid
+	author/committer line - missing email" means that setting
+	`receive.fsck.missingEmail = ignore` will hide that issue.
++
+This feature is intended to support working with legacy repositories
+which would not pass pushing when `receive.fsckObjects = true`, allowing
+the host to accept repositories with certain known issues but still catch
+other issues.
+
 receive.unpackLimit::
 	If the number of objects received in a push is below this
 	limit then the objects will be unpacked into loose object